The present invention relates to a line switching circuit for a telephone system which is connected to a central office by a subscriber's telephone line, particularly a one wherein a plurality of telephones such as a cordless telephone are coupled to a single subscriber's telephone line.
Generally, a cordless telephone consists of a mobile unit and a base unit which are connected to a single subscriber's telephone line together with an ordinary telephone in a branch configuration. The ordinary telephone will hereinafter be referred to simply as a telephone, as distinguished from the cordless telephone. The mobile unit and the base unit of the cordless telephone are connected to each other by a radio link. It is undesirable for a call to be placed on the cordless telephone (mobile unit) while the telephone is in use, since such would interfere with a conversation being held in the latter. In light of this, an effort has been made to give priority to the telephone over the cordless telephone, so that the use of the cordless telephone may be inhibited while the telephone is in use Conference Europeene des Administrations des Postes and des Telecommunication (CEPT) standards. This scheme may be implemented with two different approaches, i.e., one which cuts off the line associated with the cordless telephone while the telephone is in use, and the other which allows the cordless telephone to disconnect itself from a subscriber's line upon detection of a condition of the telephone being used; the latter approach is generally employed since an ordinary telephone is not equipped with the former function.
It is desirable that means for allowing the cordless telephone to connect and disconnect itself from the subscriber's line as stated above be provided with a function of handing over a conversation from the telephone to the cordless telephone as desired. However, a problem with such a scheme is that when the telephone becomes on-hook, the cordless telephone becomes connected to the central office upon the lapse of a certain period of time and not immmediately with the result that the central office erroneously concludes that a conversation has ended, preventing the conversation from being handed over from the telephone to the cordless telephone.